1. Field of the Invention
The present invention relates generally to a sound field control system and more specifically to a sound field control system which is adapted to control resonance noise produced in an enclosed space such as the cabin or passenger compartment of an automotive vehicle or the like.
2. Description of the Prior Art
As is well known in the field of automotive engineering, the rigid panels such as the floor panel, windows, door panels, roof panel or panels etc., which define the rigid cabin structure, when subjected to given vibrational excitement tend to vibrate in a manner that the cabin acts as a resonance chamber and produces a resonance or so called "booming" noise therein upon the frequency of the applied vibration reaching given levels.
In an effort to prevent this phenonmenon occuring during frequently used modes of vehicle operation (e.g. cruising), various passive measures such as the inclusion of sound damping materials, thicker and more rigid elastomeric glass support members for the windshield and other windows of the cabin and the like, have been employed. However, these measures have met with only limited success and simultaneously caused a notable increase in weight and cost of the vehicle.
FIG. 1 of the drawings shows an arrangement (disclosed in Japanese Patent Application Pre Publication No. Sho 48-82304) for actively suppressing noise produced in an enclosed space in response to the operation of a blower device associated therewith. In this arrangement a microphone 1 is disposed in the duct 2 interconnecting the blower 3 and the outlet port 4 and arranged to detect undesirable noise. A circuit 5 connected with the microphone 1 appropriately shifts the phase of the signal outputted by the microphone 1 and applies an energizing signal to a speaker 6 also disposed in the duct 2. As best seen in FIG. 2 of the drawings (which schematically illustrates the arrangement shown in FIG. 1), the sound waves produced by the speaker 2 are such as to cancel the waves which would otherwise produce an undesirable noise and thus silences the operation of the device.
However, when such an arrangement has been applied to the cabin of an automotive vehicle, for example, the result achieved has not been satisfactory.